The notion of stability is central to the definition of personality traits, which are generally thought of as enduring tendencies or habitual patterns of behavior, thoughts, and emotions (McCrae & Costa, 2003), but stability does not imply immutability. Under normal circumstances, adult traits are largely stable, as indicated by high correlation coefficients computed for a group assessed twice on the same trait. These coefficients represent the average stability for a sample, but individuals vary in terms of their intra-individual stability. A longitudinal intra-individual approach was used to examine stabilities across two successive intervals traced within individuals. Individual stability coefficients from eight different longitudinal samples were computed to evaluate intra-individual (i.e., within-person) change in stability over time. Test-retest periods ranged from five to nine years. For both trait and profile (ipsative) stability, results indicate that intra-individual stability increases up to age 30 and then plateaus. Neither demographic variables (sex, ethnicity, education, and secular trends), nor the standing on the five major dimensions of personality, were predictors of change in trait stability. Contrary to results from studies of adolescents, personality maturity was unrelated to personality stability in adulthood. These findings support the notion that personality stability plateaus early in adulthood. Despite the relative stability of individual differences, in recent years we have extended the studies of mean-level change in personality by examining longitudinal trajectories in more diverse samples and cross-sectional trends across cultures. We have also focused on community or epidemiological investigations to study the causes of the modest changes that occur in personality traits in adulthood. In addition to health, we are interested in the reciprocal influences of personality and life circumstances, such as job characteristics and retirement transition. Furthermore, we aim to delineate the neural and cognitive correlates of personality dimensions across longitudinal assessments. Most recently, we have investigated the association between personality factors and risk for Alzheimer's disease, as well as personality in relation to the presence of Alzheimer's pathology at autopsy. These studies have shown that high neuroticism and low conscientiousness are predictors of increased risk for Alzheimer's disease. In addition, individuals who are low in neuroticism and high in conscientiousness are more likely to be asymptomatic despite the presence of Alzheimer's pathology. As part of a larger research program in downstream consequences of certain personality traits, we investigated the role of culture, and the socioeconomic correlates of culture, on personality, health, and well-being of individuals of different age groups. We collected both self-report and personality perceptions of people across the lifespan about targets of different ages. Both the levels and trajectory of personality differences across the lifespan were substantially similar across a wide variety of cultures; perceptions about the personality of people in various age groups closely track the self-reported personality levels at each particular age. Currently we are investigating possible socioeconomic correlates of individual differences both within and across cultures in order to understand basic processes that affect the environmental expression of various trait dispositions.